1. Field of the Invention
The present invention relates to a frequency modulator comprising a self-oscillating oscillator which is modulatable in its frequency and which is stabilized with respect to its output frequency by means of a reference oscillation by way of a phase control loop having a phase indicator and a loop filter.
2. Description of the Prior Art
Frequency modulators of the type generally set forth above are known, for example from the German Pat. No. 2,055,998 (or equivalent British Pat. No. 1,371,914). The output frequency of the self-oscillating or free-running oscillator can be stabilized in the desired manner by means of a phase control loop, in conjunction with a highly-stable reference oscillation as is supplied, for example, by a crystal oscillator. The phase control loop is designed in such a manner that it cannot follow the frequency change of the modulation signal, i.e., the modulation of the free-running oscillator is thereby not prejudiced.
Such a frequency modulator is not suited for a keyed modulation as is undertaken, for example, in the transmission of telegraphic characters and teleprinter characters by way of short-wave transmitters. In such a keyed modulation application, in particular d.c. components must be transmitted when no change of state of the signal occurs over longer times. The transmission of such d.c. components, however, is in opposition to the phase control loop since, of course, it must exhibit a finite control velocity which becomes effective here and therefore levels such d.c. components.